An automobile bumper usually has an exterior surface skin, a cushioning body for shock absorption, and a reinforcement to reduce deformation. The cushioning body is an important element, and it must have high impact resistance.
The conventional bumper has the cushioning body 21 made of synthetic resin such as expanded urethane, polystyrene, or polypropylene, as shown in FIG. 8. This cushioning body 21 is filled in a space between a surface skin 1 of a synthetic resin (such as polyurethane an polypropylene) and a metal. reinforcement 2 fastened to the skin 1 by bolts 4. As shown, the bumper includes stays 3 adapted for attachment to the car body.
The conventional bumper has a disadvantage in that it does not smoothly absorb frontal shocks, shown by an arrow, and thus the shocks are transmitted to the car body. This results from fine cells of synthetic resin foam becoming compressed upon receiving shocks so that the cushioning effect of the cells diminishes as the cells become more compressed, as shown in FIG. 9.
A possible solution to eliminate this disadvantage and to provide a continuous cushioning effect is to shape the cushioning body 22 as a hollow box having a simple rectangular section, as shown in FIG. 6, so that the hollow box absorbs shocks as it is gradually flattened. However, this hollow box has a limitation in that while the cushioning effect remains as the box is compressed, the rebound power of the cushioning body abruptly drops because the body wall buckles during compression. This leads to uneven energy absorption.